1. National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568
2. Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France
3. II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany
4. Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577